This Core is intended to: 1. permit rapid and safe exchange of germline mutant mice between La Jolla and Osaka as frozen embryos, circumventing the delay imposed by quarantine;2. secure the valuable strains created using ENU;3. provide for the rescue of strains with low fertility and accelerate the fixation of mutations that can only be fixed by inbreeding, given that the MCMV screen is lethal to homozygotes;4. provide for the distribution of mutant strains to the scientific community at large by expanding and shipping homozygous stocks to the MMRRC. This Core will be run using techniques that we have established and have used routinely, and requires no expenditure for equipment. It will be mirrored by a comparable Core in Osaka, Japan, which will focus primarily upon items 1, 2, and 4 (above) to the exclusion of IVF. Neither Core can function in the absence of the other, since embryos frozen in one Core must be re-derived in the other, and vice versa. Hence they should each be viewed as separate but essential Cores for two of the participating groups (in La Jolla and Osaka).